You said the PE would be resolved before travel, so the air travel would not be an issue. But would a month in Europe in the rain be an issue? No one can answer that question.
The air travel would not be the primary issue. Airplane pressure is stabilized at high altitude at 11 psi which is equivalent to 8000 feet altitude. Although less than most people experience on the ground (sea level psi is 14.7 psi, and Marnie lives at 12 psi in Denver) the pressure differential should not be a problem, especially since airplane air has low humidity which makes it easier to breathe. Airplane air has oxygen added, which makes it nearly equivalent to sea level air in that respect. So except for that guy with tuberculosis coughing next to you, breathing airplane air comfortably should not be an issue for light levels of PE.
I would be more concerned about needing health care while traveling abroad. If you know how you would handle a PE problem if needed, that would resolve a larger concern.
If you wonder about whether the airlines or Canadian government would give permission, I would not ask the question.